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1 <br /> Subsurface Erpforation, Geofogic Ha;.a�d, and <br /> � ffa+uing Nope P1uue!1 Prrliminary Geotechnical Engrneering Report <br /> frtrat. Riuhington Design Reromm�ndations <br /> � 1d.0 DRAINAGE CONSIDERATIONS <br /> The ounvash sand and gravel is fairly permeable. However, the existing till soils aze relatively <br /> � impermeable, and precipitation wil! tend to perch and pond upon this strata. The site generally <br /> slopes down [o the north to the off-site Pidgeon Creek. During the earthwork portion of <br /> conswction, some method should be in place to contain any storm water runoff from <br /> � impervious surfaces and to discharge it to a suitable collection system. Therefore, prior to site <br /> w•ork and during construction, the contractor should be prepared ro provide temporary storm <br /> � water storage or disc6arge mechanisms as necessary. <br /> All retaining and footing walls should be pro�•ided with a drain at the footing elevation. Drains <br /> � should consist of rigid, perforated, PVC pipe surrounded by washed pea gravel. The level of <br /> the perforatioas in the pipe should be sa[ at the bottom of the footing at all locations and the <br /> drains should be conscruc[ed with sufficient gradient to allow graviry discharge away from the <br /> � building. In addi[ion, all retaining walls should be lined with a minimum 12-inch-thick washed <br /> eravel blatil:et provided o�•er the full height of the wall, which ties into the footing drain. Roof <br /> and surface runoff should not discharge into th:: footing drain system, but should be handled by <br /> � a separate, rigid, tighdine drair, In planr.ing, exterior grades adjacent [o walfs should be <br /> sloped downward away from the structure to achieve surface drainage. <br /> � 14.1 Srorm Water Collec[ion and Disposal Faciliw <br /> � Disposal of storm water via the storm sewer connection on the east side of F.vergreen W�y <br /> would likely be expensive. Water qualiry issues associated with direct disposal of storm water <br /> from impervious surfaces in[o the off-site Pidgeon Creek may also make conventional off-site <br /> � storm wa[er disposal prohibitive. T6erefore, we understand storm water collection and <br /> infiltration facilities are being considered for this site. The area available for storm water <br /> infilvation facilities on this property is limited. Therefore, it may no[ be feasible to infiltrate <br /> { storm water. Pond and vault wns[ruc[ion within the area of the site with deep recessiona! <br /> � outw:uh deposits will be challenging, and possibly expensive, given the small working area <br /> and close proximity to steep slopes with structures near the top of these slopes. Consideration <br /> � should be given to disposal of storm wa[er via individual dwelling infilvation venches if <br /> impervious surface area calculations determine that this is a feasible design option. <br /> � Storm water infiltration system.s may be viable at this site in areas with deep recessional <br /> outwash deposits. Our preliminary subsurface exploration suggesu the area of the site most <br /> 1 conducive to infiltration is the northwest corner and possibly the center of the properry. We <br /> were unable to explore the central portion of the property because a large stand of significant <br /> cedar trees occupies this area, and we were ordered by the City of Everett to kecp disturbance <br /> I to a minimum. Therefore, Curther exploration in this area of the site would likely require soil <br /> borings to minimize further disturbance. Based on the EPA Design Manua! for Onsite <br /> Wastewater Treatment and Dispasal Systems (1980), the on-site, gray sand with traces of silt <br /> ! Fe6ruary J1, 1001 ASSOC/ATED EIRTH SCIENCES, lNC. <br /> SG1fIN�KFJJl7l9A?�Pro/�m�JY117f9�KE�W.^-K:B Page 15 <br /> ( �%a <br />